Trace gas leak detectors are utilized to test for leaks in various sealed components. Known leak detection systems have typically utilized a mass spectrometer to separate helium from other gas species and measure the signal. Mass spectrometers are complex and require costly components, including costly vacuum pumping systems, to sustain operation. More recently, Penning cell sensors have been employed for leak detection in response to a demand in industry for lower cost products. Systems such as described in U.S. Pat. Nos. 5,325,708; 5,661,229 and 7,266,991 utilize a Penning cell sensor to measure trace gas ion current and scale the ion current to leak rate. U.S. Pat. No. 7,497,110 describes a leak detection system utilizing a Penning cell detector combined with a composite permeable membrane of a type described in U.S. Pat. No. 6,854,602. Despite different naming conventions (e.g., ion pump, gas consuming vacuum gauge, cold cathode gauge, ionization gauge), such references describe a technology essentially based on a simple Penning cell.
FIG. 1 is a cross-sectional view of a simple Penning cell 100. The Penning cell 100 consists of a tubular anode 102 with flat cathode plates 104 at either end. A magnetic field is applied axial to the anode 102 and the anode 102 is powered at some positive voltage, typically between +3,000 and +7,000 V, resulting in a plasma with an electron trap within the interior of the anode 102. Tracer gas molecules are flowed from a test component into the Penning cell 100 and ionized in the plasma generated by the electric field applied between the anode 102 and the cathode plates 104. The resulting gas ions are accelerated toward the cathode plates 104. Electrons from the gas molecules and cathode plates 104 form a negative space-charge cloud 106 that is constrained along the central cell axis. Some electrons migrate via cross-field mobility and strike the anode 102 and this electron current is what is measured, not the actual ion current. This electron current is assumed to scale with ion current, the ion current being proportional to gas pressure. All Penning cells provide a small amount of gas pumping simultaneous with electron current measurement. When gas ions impact the cathode surface they sputter metal. The sputtered metal fragments are deposited largely on the anode surface, thereby trapping (pumping) gas ions.
All Penning cell designs suffer from several intrinsic problems that limit the sensitivity and stability of measurement in a leak detector, which include the following. The plasma in a simple Penning cell is constrained by the electric and magnetic fields inside the anode to a small ellipsoidal volume centered on the anode cell axis. The number of electrons (which sustain the plasma) and the ion current in the cell (the number of ions available for measuring) are limited by space-charge affects so leak rate sensitivity is therefore limited in a Penning cell. Further, pumping speed is directly proportional to the amount of electrons stored in the plasma. Therefore, the greater the amount of electrons stored in the plasma, the greater the pumping speed. With the plasma limited to a small volume in the center of a Penning cell, pumping speed is limited. This is a significant factor in a leak detector. The speed with which a leak detector recovers from an exposure to trace gas is highly dependent on the pumping speed of the sensor cell.
Additionally, as noted above some electrons in the trap formed by the magnetic field and the cathode plates in the Penning cell migrate via cross-field mobility to strike the anode and this is the current actually measured, not the actual ion current. In a device measuring down to 10−15 amps, this adds a significant measurement error to the leak reading. A measurement of the anode current is assumed to scale with ion current proportional to gas pressure, i.e. the number of trace gas molecules ionized. In reality, when ions impact the cathode of a Penning cell, secondary electrons are generated and scattered, many of which in turn strike the anode causing a spurious signal (since it is anode current that is actually measured). Since the secondary electron scattering is a function of several variables including the ion energy, ion mass and the angle of incidence, this adds a varying measurement error to the signal as measured in a Penning cell.
Additionally, Penning cell devices are notoriously difficult to start at very low and very high pressures. At low pressure, even though high voltage is applied the plasma may not ignite so the sensor is inoperable. There are too few gas molecules in the Penning cell to be ionized and too few electrons generated during ionization to sustain the plasma. At high pressure, the mean free path for an ion is very short so ions quickly capture a free electron and become neutralized. There are again too few ions and electrons available to sustain a plasma discharge and the sensor extinguishes. Both of these pressure conditions have resulted in operating restrictions in prior art leak detectors. For example, it is typical for the operating manual of a commercial leak detector model to specify that the unit must be started periodically and permitted to pump so that the pressure will not rise so high during storage that the sensor cannot be restarted.
Additionally, the tensile stress level in a metal thin film deposited by sputtering can be extremely high. If the film does not adhere well to the substrate, i.e. the anode surface in the present context, the film will eventually fracture and eject metal particles into the plasma. These particles may become ionized in the plasma resulting in a high current spike and the plasma will be unstable for a period of time thereafter. This appears to a leak detector user as an unstable and unacceptable variation in leak measurement. It is also well known that metals suffer from embrittlement after absorbing substantial amounts of hydrogen. As hydrogen is one of the primary gases in a permeation-based leak detector, this is a significant factor that contributes to film failure. The choice of metals used for the anode and cathode must therefore be carefully made. Titanium has historically been used in commercial devices, but this is not the best choice for a sensor that is almost exclusively exposed to helium and hydrogen. In addition, a physical geometry around the cell that encourages consistent and even metal thin film growth is a significant design consideration.
Arcing is a problem that is intrinsic to Penning cells, resulting in large ion current spikes and instability of the leak rate signal. The typical Penning cell anode is a length of thin wall tubing with sharp edges at both ends. This is true of all commercial leak detectors on the market today that utilize a Penning cell sensor. The sharp edges of the cell anode operating at potentials of several thousand volts suffer from very high field gradients at the edges, which in turn results in field breakdown and electrical arcing between the anode and other internal surfaces. Once an arc occurs and a pit with sharp protrusions is left in the metal surface, smaller arcs will occur at the pit location on an intermittent basis. Each of these arcs results in highly unstable operation of the leak detector.
Another arcing problem results from formation of columnar structures as cathode material is sputtered by the ions and a thin metal film is deposited on the anode surface. Around the edges of the anode diameter, columnar structures grow on the cathode surface having a narrow cross-section, but can reach millimeters in height. This growth is commonly referred to as “whiskers” in the industry. Consequently, Penning devices are routinely “high-potted” (subjected to very high voltages) in order to proactively burn off the whiskers. Each of these whiskers produces a significant electric field concentration pointing directly at the sharp edge of the high voltage anode tube. The high electric field concentration results in an arc and a virtual explosion of the whisker. The resulting ion current spike causes significant instability for a leak detector for some time period until the electric fields and the plasma settle again.
In a common Penning cell, erosion of the cathode plates limits lifetime. Given the shape of the plasma at the center of the anode tube, sputtering and the resulting erosion of the cathode are concentrated in a small diameter at the center of the cell. This constant erosion due to sputtering eats through the cathode material, eventually exposing the vacuum chamber wall beneath the cathode material. This of course greatly reduces pumping speed and if left to continue will eventually eat through the vacuum chamber wall creating a vacuum leak. For the type of sealed Penning cell sensors used in leak detection, this means the sensor must be discarded as the erosion pit approaches the thickness of the cathode plate, adding significant cost to maintain a leak detector.
Penning cells have low pumping speed for noble gases such as helium since noble gases do not chemically bond and cannot be getter-pumped. The primary pumping mechanism is burial by metal sputtered from the cathode onto the anode as described above. Helium, having a low mass (mass 4), has a particularly low sputtering efficiency. Once helium enters a sensor of this type, the helium is pumped away very slowly. The slow pumping results in a high background helium level, which prevents further leak testing until the background can be reduced (pumped away). In most leak test operations, time of operation is a significant cost factor and hence the time lost waiting for sensor pump out is expensive.
One of the pumping mechanisms in a Penning cell is burial of ionized gas molecules in the cathode plates. Ionized gas molecules are accelerated toward the cathode and bury themselves in the structure of the cathode material. However, because the cathode is continually being sputtered away, these gas molecules will be re-liberated over time resulting in gas bursts and ion current instability. The same gas molecules must be ionized and pumped again and again. This is especially true when pumping noble gases and extensive studies have been documented regarding noble gas instabilities in a Penning cell. An effective sensor must provide highly effective pumping of noble gases.
All of the known Penning cell-based leak detector sensors utilize a permeable membrane made from some type of quartz that must be heated to several hundred degrees Celsius in order to permeate. This requires expensive power supplies as well as control electronics in a temperature feedback control loop to ensure the temperature does not “run away”. The high temperatures negatively affect both the performance and lifetime of adjacent components. It is well known in the industry that electronic components run best at the colder temperatures and fail more rapidly at high temperatures.
In view of the foregoing, there is an ongoing need for providing improved apparatus, devices and methods for leak detection, including improved sensitivity, improved stability, less complexity, and lower cost.